Sillcocks are used to supply water to the exteriors of buildings. Furthermore, garden hoses are oftentimes attached to the spouts of sillcocks, and nozzles are oftentimes attached to the garden hoses. Because the nozzles are equipped with on/off controls, sillcocks can be left in the on position, and the nozzles can be used to control the flow of water exiting the garden hoses. If there is a water pressure loss upstream of the sillcocks, however, possibly contaminated water can be forced backwardly from the garden hoses into the sillcocks. Depending on the amount of backpressure, the contaminated water may be forced through the sillcocks into the water supply of the building. Check seals have been used to inhibit backpressure from causing the possibly contaminated water to flow backwardly through the sillcocks, when there is a water pressure loss upstream of the sillcocks. However, the code requirements of many municipalities require that sillcocks be equipped to relieve backpressure, and, in doing so, prevent possibly contaminated water from being forced backwardly therethrough. Therefore, there is a need for a device for relieving backpressure in sillcocks.